Nitric oxides (NOx), mainly including nitric oxide (NO) and nitrogen dioxide (NO2), are some of the most toxic pollutants formed during combustion processes. NOx are precursors of both acid precipitation and ozone formation, and play important roles in the environment through acidification, forest damage, smog formation, damage to human health, depletion of the stratospheric ozone layer, and the greenhouse effect. Most NOx emissions come from automobiles, industrial boilers, refineries, and waste incineration plants, etc. Environmental protection and stringent emission limits both require a significant reduction of NOx emissions from stationary combustors.
Various technological approaches have been applied to NOx reduction from stationary sources. The two major categories of NOx control for stationary applications are precombustion control and post-combustion control. Precombustion control technologies include low NOx burner, overfire air (OFA) systems, exhaust gas recirculation (EGR), and more precisely controlled combustion parameters. Post-combustion treatments include aftertreatment technologies, such as selective catalytic reduction (SCR), selective noncatalytic reduction (SNCR), reburning, and the combination of these aftertreatment technologies. Among the post-combustion technologies, SNCR technology is considered to be an effective approach to reduce NOx, with reasonable capital investment and operation cost.
Since SNCR does not require a catalyst, it was developed with the purpose of solving the problems inherent with SCR technology, namely, high cost, high maintenance, and sensitivity to impurities in flue gas. Recent development shows that SNCR is a viable alternative to SCR technology. Also, SNCR systems have proven very effective on circulating fluidized bed (CFB) applications where the presence of the hot cyclone ensures adequate retention time at temperatures nearly ideal for NOx reduction.
To reduce NOx in a lean (excess O2) environment, a reductant is needed with an acceptable selectivity (not to be oxidized by O2). For SNCR, the most effective reductant up to now is ammonia (NH3). Urea [CO(NH2)2] distributed as a fine aerosol in water solution can be an alternative to NH3, as it readily decomposes into CO2 and NH3. The applicable reactions are as follows:

The typical operating temperature for SNCR is in the range of 850° to 1,100° C. If the temperature is over 1,100° C., NH3 may be oxidized to NO. On the other hand, if the temperature is below 850° C., the reaction rate between NH3 and NO is relatively too slow to be applied to NOx reduction from stationary flue gases.
Current SNCR efficiency is generally between 30-60%, depending upon the particular SNCR reactor design, which is much lower than that of SCR. Accordingly, a need exist to improve on the methods and apparatus that are employed for SNCR in order to provide a SNCR system that results in more efficient NOx removal.